After looking at your eSpace, there are a couple of things I would like to mention:

You are not using a ListBox. Instead you are using a Combo-box

A ComboBox can be set as mandatory, and a LisBox does not have that option.

You are using London Theme. As I told you on the other post, Silk UI Patterns (including Select2) were created to be used inside Silk UI Layouts (or custom Layouts that follow the same rules of a Silk UI Layout). The usage of Silk UI Patterns on London Theme is not guaranteed. That is why you have this strange behaviour when you see it on your browser:

The same Example, implemented on the Dublin Theme, would look like this: